In modern digital telecommunication switching systems voice samples are transferred within switching systems, as well as, between remotely located switching systems, as digital information. To preserve the integrity of the digital information and thus insure high quality voice communications, the telecommunication network is synchronized.
Therefore, it is a requirement for a telephone switching system to synchronize its local clock (slave) to that of the network (reference). Synchronization of the slave clock must be very precise and accurate since other remotely located switching systems may be synchronized to this switching system.
Synchronization between two clock signals is best accomplished by the use of a Phase Locked Loop (PLL) circuit. However, PLL circuits have several disadvantages.
As the reference signal changes frequency the PLL circuit follows these changes within certain limits. Problems arise when the reference signal drifts outside of these limits, or is completely lost, such as when a cable is cut. PLL
Because the amount that the PLL circuit frequency will have drifted is proportional to the time it takes to declare the reference frequency erroneous, a fast, accurate method of frequency detection is necessary. Prior to the present invention, frequency detection was primarily accomplished by the use of mono-stable vibrators.
The mono-stable vibrator would be designed to have a period greater than the period of the signal to be monitored, and the signal would constantly retrigger the mono-stable vibrator before it would time-out. This type of frequency detector can only detect if the frequency is too low; a high frequency would continue to retrigger the mono-stable vibrator within the time-out period and an alarm condition would not be detected.
Additionally, mono-stable vibrators tend to drift as a function of age and temperature which must be considered during their design. If accurate timing is needed, the discrete resistors used by the mono-stable vibrator are laser trimmed to provide an exact resistance, and thus an accurate time-out period, a very expensive process.
Accordingly, it is the objective of the present invention to provide a frequency detector circuit which rapidly detects an error condition when the frequency of the signal being monitored is beyond a predetermined limit.